The present invention relates to an arrangement for guiding and holding of print webs on rubber cylinders of rotary printing machines.
During operation of a rotary printing machine different tension conditions take place before, between and after the printing mechanisms which result in existence of a pressure differential at a respective one of the printing mechanisms. For avoiding the adjustment of the web tension differences before, between and after the printing mechanism during the printing process, or in other words for avoiding the adjustment of the pressure differential existing at each printing mechanism, it is known to offset the functionally cooperating rubber cylinders relative to one another by an angle corresponding to the particular conditions. In such a construction the print web is located so that it forms a greater or smaller angle of contact .phi. of the web on the cylinder, whereby a friction force is generated between the print web and the cylinder. Thereby, when a throughgoing passage is formed corresponding to the time of mutual alignment of two passages in two cylinders rolling over one another, a web slippage and thereby irregularities on the print are eliminated.
The angle of offset of both rubber cylinders can not be adjusted in a maximum manner, because of existing structural conditions in the printing machine, for example because the dimensions of functional groups of the machine are kept in relatively narrow limits for their adjustment in correspondence with the respective conditions. The angle of offset of both rubber cylinders cannot also be adjusted in maximum manner because of special printing phenomena, for example very high adhesion of the web on the respective rubber cylinders and thereby its uncontrollable excessive elongation. Because of the above-mentioned limits of adjustability of the angle of offset, the value of the friction force and its action also takes place within narrow limits. When the limit web tension difference absorbed by this friction force is exceeded, slippage of the print web in the zone of the throughgoing passage takes place and leads to reduction of the print quality.
The patent of the German Democratic Republic No. 431,259 discloses an arrangement for mounting the rubber fabric on the transmission cylinder of a rotary printing machine, which has technical means arranged in the passage on the cylinder and having a cover rod which is provided with a rubber coating and has a radius approximating to the radius of the cylinder, the cover rod constituted by a tension rod forming a functional unit. The mounting of the tension rod as well as of the cover rod is connected with great expense. After the insertion of the cover rod, the entire rubber cylinder including the rod must be cylindrically ground. When it is necessary to provide a post-tensioning of the rubber fabric, very expensive dismounting of the tensioning units accommodated in the cylinder passage and subsequent mounting must be performed. The insertion depth of the tension rod and the cover rod can change and a web slippage can take place, whereby the grinding process must be again repeated, so that a post-tensioning of the rubber fabric on the rubber cylinder which is built in the printing mechanism is not possible.